% Symbolic Computation of Surrogate Model Expression
% It is noted that the sequence of 'Coefficient_Set' need to be in accordance with the sequence of 'Degree_Nd_Set'.
% Meanwhile, the interval [-1,1] should be converted to the interval [a,b].
function Symbolic_Expression = Symbolic_Surrogate_Model_Expression(Coefficient_Set,Degree_Nd_Set,a,b,x,y)

	Polynomial_Array = sym(zeros(size(Coefficient_Set)));
	for index = 1 : size(Degree_Nd_Set,1)
		Polynomial_Array(index) = Numerical_Chebyshev_Polynomial(Degree_Nd_Set(index,1)-1,x)*...
			                      Numerical_Chebyshev_Polynomial(Degree_Nd_Set(index,2)-1,y);
	end
	Polynomial_Array = subs(Polynomial_Array,x,(2*x-a-b)/(b-a));				% x <- (2*x-a-b)/(b-a)
	Polynomial_Array = subs(Polynomial_Array,y,(2*y-a-b)/(b-a));				% x <- (2*y-a-b)/(b-a)	
	Symbolic_Expression = Coefficient_Set'*Polynomial_Array;
% 	vpa(Symbolic_Expression,16)													% 	This command is employed for debugging

end